(1) Field of the Invention
The present invention relates to microreactors, heat exchangers and mixers, and more particularly to apparatus and methods that utilize continuous flow techniques to induce substantial turbulence within the flow stream to provide a very high degree of mixing and/or heat transfer.
(2) Description of the Prior Art
A number of mixer designs have been employed in a variety of applications to mix fluent materials. Such applications include those in which it is desired to form an intimate mixture of two or more components, to form an emulsion, to cool or to heat a fluid, or to carry out chemical reactions. For example, in many chemical reactions mixing is desirable or necessary to introduce kinetic energy or to bring reactant molecules into reactive proximity to each other. This is especially desirable in many polymerization reactions in which repeated introduction of fresh reactants to the newly-created reaction sites of the burgeoning polymer is necessary to continue the chain formation. The increased exposure of reactants to each other can also be effective in increasing yield and reducing reaction times, permitting higher production rates and smaller reactors. Thus, in many reactions, the rate of chemical reaction is related to the rate of mixing of the reactants.
In addition or in the alternative, heat transfer into or out of a fluid may also be enhanced by mixing. In such situations, the turbulence constantly exposes new or other portions of the fluid to the interface at which heat transfer takes place. This is helpful in maintaining isothermal conditions, which can be particularly useful in many reactions, such as exothermic or endothermic reactions or reactions that must be carried out within a narrow temperature range. For example, hyperfast, low temperature polymerization must be carried out with significant agitation and at substantially isothermal conditions in order to produce a polymer of molecular weights within a narrow range.
Turbulence may be induced in continuous flow processes by static means, which affect the flow of a fluid through the mixer or reactor, or by moving agitators. Static mixers provide a number of advantages over mixers that contain a moving agitation means. For example, because separate mechanical agitation means are unnecessary, static mixers generally are lower cost and require less maintenance. In addition, they are less expensive to operate because they do not require energy input for agitation.
On the other hand, conventional static mixers also suffer from several disadvantages over mixers that employ mechanical agitation means. Perhaps most prominent of these disadvantages is the generally lower degree of agitation or turbulence produced by conventional static mixers. Thus, relatively large mixers or reactors are often needed. Moreover, the disadvantage of the less intense agitation is particularly pronounced in some applications in which even mixers with moving agitation means often do not create as much turbulence as desired. Aside from the failure to achieve the full measure of benefits associated with high turbulence as discussed above, the relatively low turbulence produced by many conventional mixers that are coupled with a heat transfer means requires the use of small conduits to increase transfer. The use of such conduits not only is expensive, but requires higher pressures to maintain acceptable flow rates.
Accordingly, a variety of mixer designs have been created in an attempt to combine the benefits of static mixers with the ability to achieve a degree of turbulence at least as great as, and perhaps even greater than, that of mixers that have moving agitators. Such static mixers typically employ such means as baffles or a series of segments of increased and decreased cross-sectional areas to induce turbulence in the flow of the fluid or combination of fluids, thereby to promote mixing.
For example, continuous flow polymerization reactors, some of which have a series of such segments, are shown in U.S. Pat. Nos. 3,609,125 and 3,697,230 to Yoshihisa Fujimoto et al. which disclose an apparatus which induces mixing not by expansion or contraction of the fluid path but by directing the fluid in a swirling motion, U.S. Pat. Nos. 3,628,918 and 4,175,169 to Beals et al. which show a polymerization reactor of alternating reaction and cooling zones wherein the mixing is accomplished by turbulent flow through constant diameter sections of tubing, and U.S. Pat. No. 3,674,740 to Vernaleken et al. which is directed to production of polycarbonate by what appears to be mere conventional flow through constant diameter tubes at a rate which associated with turbulent flow; i.e., a Reynolds number in excess of 2,000. U.S. Pat. No. 3,563,710 to Dew, Jr. et al. shows a finisher for removal of volatile by-products in carrying melt condensation polymerization to completion. However, the apparatus of Dew, Jr. et al. does not mix by expanding and contracting the cross-sectional area of the flowing fluid along the length of the mixer. Rather, it induces turbulence by a swirling motion of the mixture about rotor members identified in the patent by the numeral 2.
Various heat transfer apparatus that include sections of increased and decreased cross-sectional areas have been shown, for example, in U.S. Pat. Nos. 1,863,554; 4,270,601; 4,306,617; 4,437,513; 4,569,387 and 4,633,935. Of course, in heat transfer apparatus, alternating sections of increased and decreased cross-sectional areas, created such as by the use of corrugated tubing, is generally known to be employed simply to increase surface area thereby to effect greater heat transfer. In such apparatus, the changes in the cross-sectional area generally are insufficient to develop an optimal degree of turbulence in the fluid passing therethrough.
Other references are directed to apparatus which have decreased cross-sectional areas in the form of orifices through which a fluid passes. In particular, U.S. Pat. No. 2,312,639 shows a device for treating plastic. The plastic is passed through a plate having a number of perforations to increase the surface area of a mass of plastic. A significant degree of mixing does not appear to be imparted thereby to the highly viscous plastic. U.S. Pat. No. 4,313,680 shows a reactor for fast reaction which a flow-deflector is included to deflect the flow 90.degree. through several orifices.
U.S. Pat. No. 3,874,643 discloses a method for conveying pulverulent or granular thermoplastic or thermosetting material while simultaneously plasticizing, mixing and homogenizing the material by passing it through a plasticizer having alternately narrow and wide tubular passages interconnected by conic sections.
U.S. Pat. Nos. 4,964,733 and 4,861,165 show a generally tubular hydrodynamic mixer having no cylindrical sections. According to these patents, the device is designed for a number of substances and applications in the paper industry are contemplated.
In any event, mixers are still being sought that can achieve even more intense turbulence than possible with conventional apparatus, particularly for continuous flow operations. It is also highly desirable that such improved turbulence be achieved by a static mixer, especially one of relatively small size.